A method for production of methyl methacrylate by oxidative esterification using a heterogeneous catalyst

ABSTRACT

A method for preparing methyl methacrylate from methacrolein and methanol. The method comprises contacting a mixture comprising methacrolein, methanol and oxygen with a heterogeneous catalyst comprising a support and a noble metal, wherein said catalyst has an average diameter of at least 200 microns and at least 90 wt % of the noble metal is in the outer 70% of catalyst volume, and wherein oxygen concentration at a reactor outlet is from 0.5 to 7.5 mol %.

BACKGROUND OF THE INVENTION

The invention relates to a method for preparing methyl methacrylate frommethacrolein and methanol using a heterogeneous catalyst.

Heterogeneous catalysts having noble metals concentrated in an outerregion of the catalyst are known for use in oxidative esterificationreactions, see, e.g., U.S. Pat. No. 6,228,800. However, there is a needfor larger catalyst particles with noble metals more highly concentratednear the surface to provide improved selectivity.

SUMMARY OF THE INVENTION

The present invention is directed to a method for preparing methylmethacrylate from methacrolein and methanol; said method comprisingcontacting in a reactor a mixture comprising methacrolein, methanol andoxygen with a heterogeneous catalyst comprising a support and a noblemetal, wherein said catalyst has an average diameter of at least 200microns and at least 90 wt % of the noble metal is in the outer 70% ofcatalyst volume, and wherein oxygen concentration at a reactor outlet isfrom 0.5 to 7.5 mol %.

DETAILED DESCRIPTION OF THE INVENTION

All percentage compositions are weight percentages (wt %), and alltemperatures are in ° C., unless otherwise indicated. A noble metal isany of gold, platinum, iridium, osmium, silver, palladium, rhodium andruthenium. More than one noble metal may be present in the catalyst, inwhich case the limits apply to the total of all noble metals. The“catalyst center” is the centroid of the catalyst particle, i.e., themean position of all points in all coordinate directions. A diameter isany linear dimension passing through the catalyst center and the averagediameter is the arithmetic mean of all possible diameters. The aspectratio is the ratio of the longest to the shortest diameters.

Preferably, the support is a particle of an oxide material; preferablyγ-, δ-, or θ-alumina, silica, magnesia, titania, zirconia, hafnia,vanadia, niobium oxide, tantalum oxide, ceria, yttria, lanthanum oxideor a combination thereof; preferably γ-, δ-, or θ-alumina. Preferably,in portions of the catalyst comprising the noble metal, the support hasa surface area greater than 10 m²/g, preferably greater than 30 m²/g,preferably greater than 50 m²/g, preferably greater than 100 m²/g,preferably greater than 120 m²/g. In portions of the catalyst whichcomprise little or no noble metal, the support may have a surface arealess than 50 m²/g, preferably less than 20 m²/g.

Preferably, the aspect ratio of the catalyst particle is no more than10:1, preferably no more than 5:1, preferably no more than 3:1,preferably no more than 2:1, preferably no more than 1.5:1, preferablyno more than 1.1:1. Preferred shapes for the catalyst particle includespheres, cylinders, rectangular solids, rings, multi-lobed shapes (e.g.,cloverleaf cross section), shapes having multiple holes and “wagonwheels;” preferably spheres. Irregular shapes may also be used.

Preferably, at least 90 wt % of the noble metal(s) is in the outer 60%of catalyst volume (i.e., the volume of an average catalyst particle),preferably the outer 50%, preferably the outer 40%, preferably the outer35%, preferably in the outer 30%, preferably in the outer 25%.Preferably, the outer volume of any particle shape is calculated for avolume having a constant distance from its inner surface to its outersurface (the surface of the particle), measured along a lineperpendicular to the outer surface. For example, for a sphericalparticle the outer x % of volume is a spherical shell whose outersurface is the surface of the particle and whose volume is x % of thevolume of the entire sphere. Preferably, at least 95 wt % of the noblemetal is in the outer volume of the catalyst, preferably at least 97 wt%, preferably at least 99 wt %. Preferably, at least 90 wt % (preferablyat least 95 wt %, preferably at least 97 wt %, preferably at least 99 wt%) of the noble metal(s) is within a distance from the surface that isno more than 30% of the catalyst diameter, preferably no more than 25%,preferably no more than 20%, preferably no more than 15%, preferably nomore than 10%, preferably no more than 8%. Distance from the surface ismeasured along a line which is perpendicular to the surface.

Preferably, the noble metal is gold or palladium, preferably gold.

Preferably, the average diameter of the catalyst particle is at least300 microns, preferably at least 400 microns, preferably at least 500microns, preferably at least 600 microns, preferably at least 700microns, preferably at least 800 microns; preferably no more than 30 mm,preferably no more than 20 mm, preferably no more than 10 mm, preferablyno more than 7 mm, preferably no more than 5 mm. The average diameter ofthe support and the average diameter of the final catalyst particle arenot significantly different.

Preferably, the amount of noble metal as a percentage of the noble metaland the support is from 0.2 to 5 wt %, preferably at least 0.5 wt %,preferably at least 0.8 wt %, preferably at least 1 wt %, preferably atleast 1.2 wt %; preferably no more than 4 wt %, preferably no more than3 wt %, preferably no more than 2.5 wt %.

Preferably, the catalyst is produced by precipitating the noble metalfrom an aqueous solution of noble metal salt in the presence of thesupport. In one embodiment of the invention, the catalyst is produced byincipient wetness in which an aqueous solution of a suitable noble metalprecursor salt is added to a porous inorganic oxide such that the poresare filled with the solution and the water is then removed by drying.The resulting material is then converted into a finished catalyst bycalcination, reduction, or other pre-treatments known to those skilledin the art to decompose the noble metal salts into metals or metaloxides. Preferably, a C₂-C₁₈ thiol comprising at least one hydroxyl orcarboxylic acid substituent is present in the solution. Preferably, theC₂-C₁₈ thiol comprising at least one hydroxyl or carboxylic acidsubstituent has from 2 to 12 carbon atoms, preferably 2 to 8, preferably3 to 6. Preferably, the thiol compound comprises no more than 4 totalhydroxyl and carboxylic acid groups, preferably no more than 3,preferably no more than 2. Preferably, the thiol compound has no morethan 2 thiol groups, preferably no more than one. If the thiol compoundcomprises carboxylic acid substituents, they may be present in the acidform, conjugate base form or a mixture thereof. Especially preferredthiol compounds include thiomalic acid, 3-mercaptopropionic acid,thioglycolic acid, 2-mercaptoethanol and 1-thioglycerol, including theirconjugate bases.

In one embodiment of the invention, the catalyst is produced bydeposition precipitation in which a porous inorganic oxide is immersedin an aqueous solution containing a suitable noble metal precursor saltand that salt is then made to interact with the surface of the inorganicoxide by adjusting the pH of the solution. The resulting treated solidis then recovered (e.g. by filtration) and then converted into afinished catalyst by calcination, reduction, or other pre-treatmentsknown to those skilled in the art to decompose the noble metal saltsinto metals or metal oxides.

The process for producing methyl methacrylate (MMA) comprises treatingmethacrolein with methanol and oxygen in an oxidative esterificationreactor (OER). Preferably, the catalyst particles are in a catalyst bedand preferably are held in place by solid walls and by screens orcatalyst support grids. In some configurations, the screens or grids areon opposite ends of the catalyst bed and the solid walls are on theside(s), although in some configurations the catalyst bed may beenclosed entirely by screens. Preferred shapes for the catalyst bedinclude a cylinder, a rectangular solid and a cylindrical shell;preferably a cylinder. The liquid phase may further comprise byproducts,e.g., methacrolein dimethyl acetal (MDA) and methyl isobutyrate (MIB).Preferably, the liquid phase is at a temperature from 40 to 120° C.;preferably at least 50° C., preferably at least 60° C.; preferably nomore than 110° C., preferably no more than 100° C. Preferably, thecatalyst bed is at a pressure from 0 to 2000 psig (101 kPa to 14 MPa);preferably no more than 2000 kPa, preferably no more than 1500 kPa.Preferably, the catalyst bed is in a tubular continuous reactor or acontinuous stirred tank reactor. Preferably, the catalyst bed furthercomprises oxygen gas.

The OER typically produces MMA, along with methacrylic acid andunreacted methanol. Preferably, methanol and methacrolein are fed to thereactor containing the catalyst bed in a methanol:methacrolein molarratio from 1:10 to 100:1, preferably from 1:2 to 20:1, preferably from1:1 to 10:1. Preferably, the catalyst bed further comprises inertmaterials located below and/or above the catalyst. Preferred inertmaterials include, e.g., alumina, clay, glass, silica carbide andquartz. Preferably the inert material has an average diameter equal toor larger than that of the catalyst. Preferably, the reaction productsare fed to a methanol recovery distillation column which provides anoverhead stream rich in methanol and methacrolein; preferably thisstream is recycled back to the OER. The bottoms stream from the methanolrecovery distillation column comprises MMA, MDA, methacrylic acid, saltsand water. In one embodiment of the invention, MDA is hydrolyzed in amedium comprising MMA, MDA, methacrylic acid, salts and water. MDA maybe hydrolyzed in the bottoms stream from a methanol recoverydistillation column; said stream comprising MMA, MDA, methacrylic acid,salts and water. In another embodiment, MDA is hydrolyzed in an organicphase separated from the methanol recovery bottoms stream. It may benecessary to add water to the organic phase to ensure that there issufficient water for the MDA hydrolysis; these amounts may be determinedeasily from the composition of the organic phase. The product of the MDAhydrolysis reactor is phase separated and the organic phase passesthrough one or more distillation columns to produce MMA product andlight and/or heavy byproducts. In another embodiment, hydrolysis couldbe conducted within the distillation column itself.

One preferred embodiment is a recycle reactor with cooling capacity inthe recycle loop. Another preferred embodiment is a series of reactorswith cooling and mixing capacity between the reactors.

Preferably, oxygen concentration at a reactor outlet is at least 1 mol%, preferably at least 2 mol %, preferably at least 3 mol %; preferablyno more than 7 mol %, preferably no more than 6.5 mol %, preferably nomore than 6 mol %. Preferably, the superficial velocity of liquidthrough the reactor is from 1 to 50 mm/s, preferably at least 2 mm/s,preferably at least 3 mm/s, preferably at least 4 mm/s, preferably atleast 5 mm/s; preferably no more than 40 mm/s, preferably no more than25 mm/s.

In a preferred embodiment of the invention, pH at the reactor outlet isfrom 3 to 6.7; preferably at least 3.5, preferably at least 4,preferably at least 4.5, preferably at least 4.8, preferably at least 5;preferably no more than 6.6, preferably no more than 6.5, preferably nomore than 6.4, preferably no more than 6.3, preferably no more than 6.2.Preferably, base is not added to the reactor or to liquid streamsentering the reactor. Preferably, the reactor is not connected to anexternal mixing tank through which base is introduced. pH in the reactoris likely to be higher, possibly above 7 near the inlet and droppingbelow 6 at the outlet.

One preferred embodiment of the fixed bed reactor for oxidativeesterification is a trickle bed reactor, which contains a fixed bed ofcatalyst and passes both the gas and liquid feeds through the reactor inthe downward direction. In trickle flow, the gas phase is the continuousfluid phase. Thus, the zone at the top of the reactor, above the fixedbed, will be filled with a vapor phase mixture of nitrogen, oxygen, andthe volatile liquid components at their respective vapor pressures.Under typical operating temperatures and pressures (50-90° C. and 60-300psig (510-2160 kPa), this vapor mixture is inside the flammable envelopeif the gas feed is air. Thus, only an ignition source would be requiredto initiate a deflagration, which could lead to loss of primarycontainment and harm to the physical infrastructure and personnel in thevicinity. In order to address process safety considerations, a means tooperate a trickle bed reactor while avoiding a flammable headspaceatmosphere is operation with a gas feed containing a sufficiently lowoxygen mole fraction to ensure the oxygen concentration in the vaporheadspace is below the limiting oxygen concentration (LOC).

Knowledge of the LOC is required for the fuel mixture, temperature, andpressure of concern. Since the LOC decreases with increasing temperatureand pressure, and given that methanol gives a lower LOC than the othertwo significant fuels (methacrolein and methyl methacrylate), aconservative design chooses a feed oxygen to nitrogen ratio that ensuresa composition with less than the LOC at the highest expected operatingtemperature and pressure. For example, for a reactor operated at up to100° C. and 275 psig (1990 kPa), the feed oxygen concentration innitrogen should not exceed 7.4 mol %.

EXAMPLES

Examples of the effect of vent O₂ concentration are provided below.

Example 1

A series of runs was conducted in which 20 wt % methacrolein, 200 ppminhibitor, and a balance of methanol were fed to a ⅜″ (9.5 mm) stainlesssteel tubular reactor containing a short front section of silica carbidefollowed by 10 g of catalyst. The catalyst consisted of 1.5 wt % Au on aNorpro 1 mm diameter high surface area alumina spherical support. The Auis primarily located near the outer edge of the catalyst within 150microns of the outer edge. A gas containing 8% oxygen in nitrogen wasalso feed to the reactor. The reactor was operated at 60° C. and 160psig (1200 kPa). The product of the reactor was sent to a liquid-vaporseparator and the vapor was sent to a condenser with liquid return. Aportion of the product stream from this separator was recycled in somecases to the reactor inlet and combined with the feed entering thereactor. Results are described in the below table. MIB is reported inppm on a 100% MMA product basis. Product MMA is the percent MMA amongproducts originating as methacrolein reactant.

Prod Vent MMA Cony MIB Feed Recycle O₂ Gas Gas (%) (%) (ppm) (g/hr)(g/hr) (mol %) (SCCM) Type 99.7 8.0 1170 100 0 0.2 75 8% O₂ 96.4 10.5670 200 0 1.9 200 8% O₂ 97.6 17.9 540 100 100 2.3 200 8% O₂ 97.9 14.4540 200 0 2.6 500 8% O₂ 98.0 24.7 470 100 100 3.1 500 8% O₂ 97.8 75.8460 20 0 4 450 8% O₂

Examples of the effect of eggshell vs uniform catalyst loading areprovided below.

Catalysts

Catalysts tested consisted of one egg-shell type catalyst and one withapproximately uniform Au loading. The egg-shell catalyst consisted ofapproximately 1.5 wt % Au loaded primarily on the outer 50 microns of aNorpro 3.2 mm high-surface area alumina support material. The uniformlyloaded catalyst consisted of approximately 1.5 wt % Au loaded on thesame type of Norpro 3.2 mm alumina support, but in an approximatelyuniform loading.

Catalyst Testing

Catalysts were evaluated in a continuous fixed-bed reactor operated intrickle flow mode. In each case, approximately 0.5 g of catalyst wasmixed with silicon carbide grit to ensure uniform wetting. The catalystbed was sandwiched between layers of glass beads. The reactor wasoperated at 60° C. and 160 psig with an inlet oxygen composition of 6(achieved with 20 sccm air and 50 sccm He) or 21 mol % 02 at a gas flowrate of 70 sccm. Liquid feed (10 wt % methacrolein in methanol) wasintroduced at a flow rate of 0.07 mL/min. Performance over time, MMArate as a space time yield, and MIB content (ppm on a 100% MMA basis)are shown in the table below. At higher oxygen concentrations in thefeed, the MIB values begin to converge consistent with the hypothesisthat barriers to oxygen diffusion generally correlate with higherproduction of this impurity.

Space-Time Product Oxygen level Yield MIB on a fed to reactor [mol MMA/100% MMA [mol %] kgcat/hr] basis [ppm] Egg-shell metal 6 4 504 loadingEgg-shell metal 21 4 423 loading Predominately 6 1 740 uniform metalloading Predominately 21 2 405 uniform metal loading

1. A method for preparing methyl methacrylate from methacrolein andmethanol; said method comprising contacting a mixture comprisingmethacrolein, methanol and oxygen with a heterogeneous catalystcomprising a support and a noble metal, wherein said catalyst has anaverage diameter of at least 200 microns and at least 90 wt % of thenoble metal is in the outer 70% of catalyst volume, and wherein oxygenconcentration at a reactor outlet is from 0.5 to 7.5 mol %.
 2. Themethod of claim 1 in which the catalyst has an average diameter from 400microns to 10 mm.
 3. The method of claim 2 in which the catalyst iscontained in a catalyst bed.
 4. The method of claim 3 in which thecatalyst bed is at a temperature from 40 to 120° C.
 5. The method ofclaim 4 in which pH in the catalyst bed is from 4 to
 10. 6. The methodof claim 5 in which at least 90 wt % of the noble metal is in the outer50% of catalyst volume.
 7. The method of claim 6 in which the noblemetal is selected from the group consisting of gold and palladium. 8.The method of claim 7 in which the support is selected from the groupconsisting of γ-, δ-, or θ-alumina.
 9. The method of claim 8 in whichmethanol and methacrolein are fed to a reactor containing the catalystbed in a molar ratio from 1:1 to 10:1, respectively.
 10. The method ofclaim 9 in which at least 95 wt % of the noble metal is in the outer 40%of catalyst volume.